Atoms, bonding and types of reactions

For Chromium, you would expect Cr: [Ar] 3d⁴4s², however it is actually Cr: [Ar] 3d⁵4s¹. Similarly for Copper one would expect Cu: [Ar] 3d⁹4s², however it is actually Cu: [Ar] 3d¹⁰4s¹.

Some people try to explain the break in pattern by claiming that half-filled or completely-filled d orbitals are more stable than a completely-filled s-orbital. For the purposes of A-level examinations this is a satisfactory explanation, however this is not actually the case! Continue to the Stretch & Challenge section if you wish to learn more.

Continuing from Chromium:

Cr: [Ar] 3d⁵4s¹

Mn: [Ar] 3d⁵4s²

Fe: [Ar] 3d⁶4s²

Co: [Ar] 3d⁷4s²

Ni: [Ar] 3d⁸4s²

Cu: [Ar] 3d¹⁰4s¹

Zn: [Ar] 3d¹⁰4s²

Remember, electrons will fill available orbitals singly (spin aligned) before pairing up (spin paired). This is illustrated for d⁵ and d⁸: